In starting a high intensity discharge (HID) lamp, the lamp experiences three phases. These phases include breakdown, glow discharge, and thermionic arc. Breakdown requires a high voltage to be applied between the electrodes of the lamp. Following breakdown, the voltage must be high enough to sustain a glow discharge and heat the electrode to thermionic emission. Once thermionic emission commences, current must be maintained in the run-up phase until the electrodes reach a steady-state temperature. After achieving the arc state, the lamp can be operated with a lower level of current in the steady state operating mode.
For ignition of the lamp, the lamp must be provided with a high voltage for a specified duration in the pre-breakdown period. Conventional lamps are characterized by a minimum voltage level and time duration in achieving breakdown. HID lamps require a high ignition voltage (e.g., 1000 to 5000 Vrms) to initiate the plasma discharge when cold. Lamp input power is typically 5–10 times higher during lamp ignition than the rated steady state lamp power because of high transient power losses. This voltage creates a high intensity electrical field applied to the electrodes that initiates the discharge. The high voltage requirements for breakdown can be achieved through pulse resonant circuits. The frequency at which the circuit achieves resonance and the resultant resonant voltage varies from circuit to circuit due to variation in component tolerances. Because lamp starting voltage depends on inverter input voltage, it is important that the DC bus voltage is maintained by keeping it in a definite range as long as possible before the lamp ignites. Once the arc has been established, it is beneficial to provide a constant power to the lamp to assure a constant and reliable light output.
Typically, electronic ballasts regulate lamp power when operating high intensity discharge lamps by sensing the lamp current and the lamp voltage. The sensed lamp current and voltage are multiplied to get the wattage. The multiplication can be achieved using a micro-controller or microprocessor. The wattage is then compared to a reference wattage. A feedback loop is provided in such a way that the error that results from this comparison is converted to a signal adjusting the lamp current so that the measured lamp power is equal to the reference power.
Prior art electronic ballasts for HID lamps receive an alternating line current, such as the alternating line current provided by a voltage source 10 as shown in FIG. 1. The current is provided to a rectifier circuit 12, which generates an output to a boost converter 14. The boost converter is typically controlled by a power factor correction controller 16. The output of the boost converter typically has it own voltage control loop to maintain its output voltage higher than the input voltage. The boost converter is followed by a power processing stage comprising a DC-DC converter 18, such as a buck converter or other suitable type of DC-DC converter, that again has its own control loop, such as a pulse width modulation (PWM) controller 20, and is used to maintain a constant voltage or current output and to perform the necessary voltage conversion and conditioning. The power processing stage is coupled to an inverter 22 (controlled by a corresponding inverter driver circuit 24) which delivers power to the lamp 26.
However, the additional power processing stage results in additional power losses and requires additional components which lead to increased size and higher cost. In manufacturing electronics generally, any reduction in the necessary parts can be significant. In the field of electronic ballasts, any improvement which can reduce material cost is significant. For example, the reduction or elimination of conventional circuitry can reduce part count and reduce cost significantly. Therefore, a need exists for a ballast that does not require a separate power processing stage in order to regulate the power that is supplied to an HID lamp.